Inter- and Intramolecular Proton Transfer in an Isolated (Cytosine-Guanine)H+ Pair: Direct Evidence from IRMPD Spectroscopy.

The collision-induced dissociation of the protonated cytosine-guanine pair was studied using tandem mass spectrometry (MS3) coupled to infrared multiple photon dissociation spectroscopy with the free electron laser at Orsay (CLIO) to determine the structure of the CH+ and GH+ ionic fragments. The results were rationalized with the help of electronic structure calculations at the density functional theory level with the B3LYP/6-311++G(3df,2p) method. Several tautomers of each fragment were identified for the first time, some of which were previously predicted by other authors. In addition, two unexpected and minor tautomers were also found: cytosine keto-imino [CKI(1,2,3,4)H+] and guanine keto-amino [GKA(1,3,7)H+]. These results highlight the importance of the DNA base tautomerization assisted by inter- and intramolecular proton or hydrogen transfer within the protonated pairs.

Guanine Tautomerism in Ionic Complexes with Ag+ Investigated by IRMPD Spectroscopy and Mass Spectrometry.

In this paper, we present the IRMPD spectra of three ionic complexes between guanine (G) and silver (Ag+): [GAg-H2O]+, [GAgG]+ produced in the electrospray ionization source of the mass spectrometer, and [GAg]+ produced by collision induced dissociation of the [GAgG]+ complex. On the basis of the comparison of theoretically calculated IR spectra, we show that there are two isomers of each complex containing two different keto-amino (KA) tautomers of G (GKA(1,9) and GKA(1,7)). The observed isomers are the most stable structures in aqueous solution, and their experimentally estimated relative populations are in better agreement with the calculated relative populations in solution than in the gas phase, both at 298 K. We concluded that these observations suggest that GKA(1,9) and GKA(1,7) coexist in solution according to previous theoretical reports (Colominas, C.; et al. J. Am. Chem. Soc. 1996, 118, 6811). We were unable to find any evidence of the presence of the GEA(9), GKA(3,7), GKA(3,9), or GKA(7,9), whose relative stabilities in solution are strongly dependent on the theoretical method used to account for the solvent effect (Hanus, M.; et al. J. Am. Chem. Soc. 2003, 125, 7678).

Infrared Multiple Photon Dissociation Spectroscopy of Protonated Cyameluric Acid.

The present study reports the first structural characterization of protonated cyameluric acid ([CA + H]+) in the gas phase, which paves the way for prospective bottom-up research on the condensed-phase chemistry of CA in the protonated form. A number of [CA + H]+ keto-enol isomers can a priori be produced as a result of protonation at available N and O positions of precursor neutral CA tautomers, yet ab initio computations predict different reduced [CA + H]+ isomer populations dominating the solution and gas phases that are involved in the ion generation process (i.e., electrospray ionization). Infrared multiple photon dissociation spectra were recorded in the 990-1900 and 3300-3650 cm-1 regions and compared with theoretical [B3LYP/6-311++G(d,p)] IR absorption spectra of several [CA + H]+ isomers, providing a satisfactory agreement for the most stable monohydroxy form in the gas phase, [1358a]+, yet the contribution of its nearly isoenergetic OH rotamer, [1358b]+, cannot be neglected. This is indicative of the occurrence of [CA + H]+ isomer interconversion reactions, assisted by protic solvent molecules, during their transfer into the gas phase. The results suggest that available O positions on neutral CA are energetically favored protonation sites in the gas phase.

On the Interaction between Deprotonated Cytosine [C(-H) ]- and Ba2+ : Infrared Multiphoton Spectroscopy and Dynamics.

Gas-phase interactions between Ba2+ and deprotonated cytosine (C(-H) ) were studied in [C(-H) Ba]+ and [C(-H) BaC]+ complexes by IRMPD spectroscopy coupled to tandem mass-spectrometry in combination with DFT calculations. For the [C(-H) BaC]+ complex only one [C(-H) KAN1O-Ba-Canti ]+ isomer was found, although the presence of another structure cannot be excluded. This isomer features a central tetracoordinated Ba2+ that simultaneously interacts with keto-amino [C(-H) ]- deprotonated on N1 and neutral keto-amino C. Both moieties are in different planes as a consequence of an additional NH…O=C hydrogen bond between C and [C(-H) ]- . A sequential IRMPD dynamics is observed in this complex. For the [C(-H) Ba]+ complex produced by electrospray ionization two isomers ([C(-H) KAN1OBa]+ and [C(-H) KAN3OBa]+ ) were identified, in which Ba2+ interacts simultaneously with the C=O group and the N1 or N3 atom of the keto-amino [C(-H) ]- , respectively. A comparison with the related [C(-H) Pb]+ complex (J. Y. Salpin et al., Chem. Phys. Chem. 2014, 15, 2959-2971) is also presented.

UV Photofragmentation of Cold Cytosine-M+ Complexes (M+: Na+, K+, Ag+).

The UV photofragmentation spectra of cold cytosine-M+ complexes (M+: Na+, K+, Ag+) were recorded and analyzed through comparison with geometry optimizations and frequency calculations of the ground and excited states at the SCS-CC2/Def2-SVPD level of theory. While in all complexes, the ground state minimum geometry is planar (Cs symmetry), the ππ* state minimum geometry has the NH2 group slightly twisted and an out-of-plane metal cation. This was confirmed by comparing the simulated ππ* Franck-Condon spectra with the vibrationally resolved photofragmentation spectra of CytNa+ and CytK+. Vertical excitation transitions were also calculated to evaluate the energies of the CT states involving the transfer of an electron from the Cyt moiety to M+. For both CytK+ and CytNa+ complexes, the first CT state corresponds to an electron transfer from the cytosine aromatic π ring to the antibonding σ* orbital centered on the alkali cation. This πσ* state is predicted to lie much higher in energy (>6 eV) than the band origin of the π-π* electronic transition (around 4.3 eV) unlike what is observed for the CytAg+ complex for which the first excited state has a nOσ* electronic configuration. This is the reason for the absence of the Cyt+ + M charge transfer fragmentation channel for CytK+ and CytNa+ complexes.

Fingerprints of Both Watson-Crick and Hoogsteen Isomers of the Isolated (Cytosine-Guanine)H+ Pair.

 Gas phase protonated guanine-cytosine (CGH+) pair was generated using an electrospray ionization source from solutions at two different pH (5.8 and 3.2). Consistent evidence from MS/MS fragmentation patterns and differential ion mobility spectra (DIMS) point toward the presence of two isomers of the CGH+ pair, whose relative populations depend strongly on the pH of the solution. Gas phase infrared multiphoton dissociation (IRMPD) spectroscopy in the 900-1900 cm-1 spectral range further confirms that the Watson-Crick isomer is preferentially produced (91%) at pH = 5.8, while the Hoogsteen isomer predominates (66%) at pH = 3.2). These fingerprint signatures are expected to be useful for the development of new analytical methodologies and to trigger isomer selective photochemical studies of protonated DNA base pairs.